1. Field of the Invention
The invention includes a method of manufacturing single-walled carbon nanotubes by promoting self-assembly of single crystals of single-walled carbon nanotubes using field enhanced thermolysis of nano-patterned precursors. With the disclosed method a higher ordering degree of the grown nanotubes than with known methods can be achieved while the synthesis of these highly ordered single crystals of single-walled carbon nanotubes results in extended structures with length dimensions on the micron scale. They are formed from nanotubes that have identical diameter and chirality within each crystal but which may differ between the crystals. With the proposed method single-walled carbon nanotubes can be produced as a highly ordered bulk material on the micron scale which is a first step for the synthesis of bulk macroscopic crystalline material. The invention hence represents a significant advance in the synthesis of crystals containing a high number of well-aligned ordered single-walled carbon nanotubes all of which are physically identical in nature.
2. Description of the Related Art
Carbon nanotubes have been the subject of intense research since their discovery in 1991. One of the most desirable aims of carbon nanotube fabrication is to form large uniform and ordered nano- and microstructures and eventually bulk materials.
The potential applications of single-walled carbon nanotubes range from structural materials with extraordinary mechanical properties down to nanoelectronic components with a potential to circumvent Moore""s Law. Single-walled carbon nanotubes can act as ultimate probe tips for scanned probe microscopy with the added ability to chemically functionalize the apex. These nanostructures are also usable for forming microbalances, gas detectors or even energy storage devices. Likewise the use of single-walled carbon nanotubes in the field emission mode for displays or as electrodes for organic light emitting diodes or for electron beam sources in lithography and microscopy are of clear future technological significance.
The growth of single-walled carbon nanotubes traditionally uses harsh conditions such as laser ablation of carbon rods or a direct current arc discharge between carbon electrodes in an inert gas environment, such as described in xe2x80x9cFullerene Nanotubes: C1,000,000 and Beyondxe2x80x9d, Yakobson and Smalley, American Scientist, Vol. 85, No. 4, July-August 1997, pp. 324-337. For both methods the addition of a small quantity of metal catalyst like Co, Ni, Fe, or Mo increases the yield of single-walled carbon nanotubes. To date the resulting material consists however only of an entangled and poorly ordered mat of single-walled carbon nanotubes although each nanotube can be several hundreds of microns long. Furthermore, a wide variation in structures referred to as the zigzag, armchair or chiral forms coexist within the material. U.S. Pat. No. 5,424,054 presents a method for manufacturing hollow fibers having a cylindrical wall comprising a single layer of carbon atoms, but also here the produced fibers have no controlled orientation.
In a recent article xe2x80x9cCarbon rings and cages in the growth of single-walled carbon nanotubesxe2x80x9d by Ching-Hwa Kiang, Journal of Chemical Physics, Vol. 113, No. 11, 15 September 2000, a growth model for single-walled carbon nanotubes is presented based on an analysis of the experimental results of arc- and laser-grown single-walled carbon nanotubes.
In xe2x80x9cGrowth of a single freestanding multiwall carbon nanotube on each nanonickel dotxe2x80x9d, by Ren et al. in Applied Physics Letters, Vol 75, No. 8, 23. August 1999, pp. 1086-1088, the use of chemical vapor deposition in combination with nanofabricated catalytic patterning or templating has been used to direct the growth of individual single-walled carbon nanotubes on substrates. However, ordered arrays beyond short sections of ordered single-walled carbon nanotubes of tens of nanotubes have not been produced. Likewise, chirality and diameter are not controllable which for many applications is of paramount importance because the physical properties of the nanotubes such as electrical conductivity are extremely structure-sensitive.
According to a first aspect of the invention there is provided a method of manufacturing single-walled carbon nanotubes comprising the steps of providing on a substrate at least one pillar comprising alternate layers of a first precursor material comprising fullerene molecules and a second precursor material comprising a catalyst, and heating the at least one pillar in the presence of a first magnetic, electromagnetic or electric field. During the heating, crystals comprising single-walled carbon nanotubes grow. The crystal growth direction is determined by the direction of the applied field. The precursor materials can be provided by thermal evaporation. As the fullerene molecules C60 or C82 molecules can be preferably used.
It proves an advantageous choice to provide the pillars to have between 5 and 10 layers of the precursor materials deposited upon each other. Each layer may have a thickness between 5 and 30 nm.
The precursor materials can be deposited through a shadow mask comprising one or more apertures. Such a shadow mask has the advantage to be suited for not only providing an aperture for creating one pillar, but with such a shadow mask a large number of such pillars can be fabricated in parallel. Furthermore the fabrication of the apertures in the shadow mask can be done in parallel as well, e.g. by a lithography process.
The substrate can be selected to comprise thermally oxidized silicon or molybdenum in the form of a grid or as a solid film provided on a silicon wafer. The substrate can also be selected to have a rough faceted surface such that it offers crystallization sites, i.e. seed locations from where the crystals respectively the nanotubes can grow.
The substrate ideally is selected to have a surface structure that helps the pillars to stay confined also during the heating step. It is found that the better the confinement of the pillars on the surface, the higher the yield in precisely aligned crystals. The substrate is optimally selected, to not, or only to a negligible extent, participate in the chemical reaction that takes place during the heating step. It is further preferable to have the property to effectively keep the pillars confined thereon. A diffusion of the pillar structure on the surface reduces the yield. Molybdenum or silicon dioxide have been found to be materials for the substrate that meet with both of the above criteria. Particularly molybdenum is found to offer through its surface structure numerous crystallization sites. Instead of a bulk substrate, any layered structure comprising different materials can be used. For the manufacturing method, the upmost layer is the one that influences the process and which herein is referred to as the substrate.
The evaporation of the precursor materials can be performed at a pressure of around 10xe2x88x929 Torr, while the substrate can be kept at room temperature. The evaporation can be controlled by using an electromechanical shutter and an in situ balance for monitoring the deposition rate of the precursor materials. The evaporation can be controlled such that the thickness of the layers decreases with their distance from the substrate. This decreasing thickness again increases the yield and it is believed that the reduction in thickness directly leads to the effect that less of the catalyst is transported towards the tip of the growing crystal. Furthermore the evaporation of a catalyst like Ni is technically not so easy which makes it desirable to utilize only the minimum necessary amount for the manufacturing process. Hence the amount of catalyst material can be reduced by the thinner layers. Since it is also believed that the growth of the crystal begins at the base of the pillar, less material transport form the layers which are remote from the substrate is performed with the layers with reduced thickness.
The heating can be performed up to a temperature of essentially 950xc2x0 C. in a vacuum of essentially 10xe2x88x926 Torr or in an essentially inert gas atmosphere, for a time between 3 minutes and an hour. Thereby better results are obtained. A heating time in the minute range is in principle seen sufficient which means that a longer heating does not significantly improve the result.
In the case of applying the first magnetic field, this magnetic field can be oriented essentially normal to the surface of the substrate during heating, such that the growing nanotubes follow the applied field and grow perpendicularly to the substrate surface as well. This field can be concentrated onto the at least one pillar being heated. This proves advantageous, when the heating source is constructed in a way that counteracts the applied field. Since at 950xc2x0 C. the Curie temperature of a magnet is exceeded, the magnetic field in that heated area would be destroyed. Keeping the magnet away from the heat source such that the heat does not harm the magnet, but directing the field to the pillars allows to keep the magnetic field effective in the pillar area. In the case of growing more than one pillar, the first magnetic field can be applied in a different orientation onto different of the pillars, thereby effecting different crystals growing into different, but controlled, directions. Even crystal intersections can be achieved in this way. After the heating has led to the growth of the single-walled carbon nanotubes, these can be thermally annealed in the presence of a second magnetic field. During this process step, the crystal direction is again determined by the direction of the applied field. When the direction of the second magnetic field differs from the direction of the first magnetic field, the crystal is redirected into the new direction determined by the second applied field.
In the case of applying an electrical field, this electrical field can be directed essentially parallel to the substrate surface in order to have the nanotubes grow orthogonally to the substrate surface.
According to another aspect of the invention a precursor arrangement for manufacturing single-walled carbon nanotubes is provided, which comprises on a substrate at least one pillar comprising alternate layers of a first precursor material comprising fullerene molecules and a second precursor material comprising a catalyst. The layers may have a thickness that decreases with their distance from the substrate. The substrate may comprise thermally oxidized silicon or molybdenum in the form of a grid or as a solid film provided on a silicon wafer. The catalyst may comprise a magnetic material, preferably a metal being selected from the group Ni, Co, Fe, Mo.
According to another aspect of the invention a nanotube arrangement is proposed comprising a substrate and thereupon at least one crystal comprising a bundle of single-walled carbon nanotubes with identical orientation and structure. The nanotube arrangement can be integrated in a display, electrical circuit, switching element or sensor element.
A further aspect of the invention is to provide a nanotube crystal comprising a bundle of straight single-walled carbon nanotubes with essentially identical orientation and structure.